The main uses for a flash are usually to fill in shadows in a subject or to separate a subject from a background. Or to illuminate a subject when the available lighting is too dark to achieve the shot you want.

But unless you have a good external flash, I would recommend against using the pop-up flash whenever possible. The light those things cast can be quite harsh and unpleasing.

Take someone outside with you on a bright, sunlit day. Have him/her slowly turn around while you circle, watching the face. See how the shadows cast by the sun fall on the face - particularly the shadow from the nose. If the person is wearing glasses, the shadows from the glasses become really noticeable at some points in the rotation.

Those shadows can be quite dark in a picture. It can result in a photograph which just isn't pleasant, because the shadows call attention to themselves.

You can use a flash unit or a reflector to 'fill in' or lighten those shadows. The trick is to lighten them, but not eliminate them entirely. The shadows are important in creating a three-dimensional feel to the final print. If the lighting is too even [flat], this sense of 3D modelling of the features is lost.

The 'ideal' flash unit is one which can be taken off the camera and held separately from the camera - high, low, or off to the side. Such a flash can be pointed in whatever direction you wish.

If your camera has a pc flash socket or a hot shoe, you can plug in a flash unit.